1. Field of the Invention
The present invention relates to a fiber-grating semiconductor, and more particularly, to a fiber-grating semiconductor laser capable of varying the output wavelength by applying stress on a fiber grating as an external resonating mirror.
2. Description of the Related Art
A wavelength-tunable semiconductor laser is emerging as a backup light source indispensable for a Wavelength Division Multiplexing (WDM)-based multi-channel optical transmission system, and studies on applications of the light source have been under way. As a result of the studies, several tunable semiconductor lasers such as a sampled grating tunable semiconductor laser, a temperature-controlled distributed feedback semiconductor laser, a temperature-controlled fiber-grating semiconductor laser and an external cavity tunable semiconductor laser have been developed. The sampled grating tunable semiconductor laser uses the non-zeroth order operation of a distributed feedback laser diode based on a sampled grating. The temperature-controlled distributed feedback semiconductor laser and the fiber-grating semiconductor laser have the output-wavelength tunability by controlling the temperatures of their grating regions. The external-cavity wavelength-tunable semiconductor laser varies the output wavelength by rotating a bulk grating as an external resonating mirror.
However, the above tunable semiconductor lasers have the following problems.
First, the sampled-grating wavelength-tunable semiconductor laser is disadvantageous in that it is not easy to control the output wavelength linearly and maintain the output light power stably when varying the wavelength.
Second, the temperature-controlled distributed feedback semiconductor laser has a competitive edge in that it is low-priced and simple-structured. However, performances of the distributed feedback semiconductor laser are dependent on temperature and the speed for varying the wavelength is not fast.
Third, a temperature-controlled fiber-grating semiconductor laser is disadvantageous in terms of the temperature stability of the fiber grating and the speed for varying the wavelength.
Fourth, since the external-cavity wavelength-tunable semiconductor laser has already been commercialized, it stays ahead of the other tunable semiconductor lasers in terms of technological advancement. However, it is too big and costly.
That is, the existing four approaches to manufacturing of the tunable semiconductor laser has limits in satisfying all the requirements for easy wavelength tuning, stability, high speed and low cost.